I. Field of the Invention
This invention relates generally to the controlled deposition of ultrathin films. In particular, the present invention relates to the layer-by-layer growth of uniform and conformal films with atomic-level control. The invention also relates to the formation of an ultrathin shell on a core particle and the incorporation of such core-shell particles in energy conversion devices.
II. Background of the Related Art
Metals such as platinum (Pt), palladium (Pd), ruthenium (Ru), and related alloys are known to be excellent catalysts. When incorporated in electrodes of an electrochemical device such as a fuel cell, these materials function as electrocatalysts since they accelerate electrochemical reactions at electrode surfaces yet are not themselves consumed by the overall reaction. Although noble metals have been shown to be some of the best electrocatalysts, their successful implementation in commercially available energy conversion devices is hindered by their high cost in combination with other factors such as a susceptibility to carbon monoxide (CO) poisoning, poor stability under cyclic loading, and the relatively slow kinetics of the oxygen reduction reaction (ORR).
A variety of approaches have been employed in attempting to address these issues. One approach involves increasing the overall surface area available for reaction by forming metal particles with nanometer-scale dimensions. Loading of more expensive noble metals such as Pt has been further reduced by forming nanoparticles from alloys comprised of Pt and a low-cost component. Still further improvements have been attained by forming core-shell nanoparticles in which a core particle is coated with a thin shell of a different material which functions as the electrocatalyst. The core is usually a low cost material which is easily fabricated whereas the shell comprises a more catalytically active noble metal. An example is provided by U.S. Pat. No. 6,670,301 to Adzic, et al. which discloses a process for depositing a thin film of Pt on dispersed Ru nanoparticles supported by carbon (C) substrates. Another example is U.S. Patent Application Publ. No. 2006/0135359 to Adzic, et al. which discloses platinum- and platinum-alloy coated palladium and palladium alloy nanoparticles. Each of the aforementioned is incorporated by reference as if fully set forth in this specification. Although these approaches have produced catalysts with a higher catalytic activity and reduced noble metal loading, still further improvements are needed for electrochemical energy conversion devices to become cost-effective alternatives to conventional fossil fuel-based devices.
Practical synthesis of core-shell particles with peak activity levels requires the development of cost-effective processes which provide atomic-level control over shell formation. Such a process needs to be able to form uniform and conformal atomic-layer coatings of the desired material on a large number of three-dimensional particles having sizes as small as a few nanometers. One method of depositing a monolayer of Pt on particles of different metals involves the initial deposition of an atomic monolayer of a metal such as copper (Cu) by underpotential deposition (UPD). This is followed by galvanic displacement of the underlying Cu atoms by a more noble metal such as Pt as disclosed, for example, in U.S. Patent Application Publ. No. 2007/0264189 to Adzic, et al. Another method involves hydrogen adsorption-induced deposition of a monolayer of metal atoms on noble metal particles as described, for example, in U.S. Pat. No. 7,507,495 to Wang, et al. Both of the aforementioned are incorporated by reference as if fully set forth in this specification.
Since both of the above methods involve the galvanic displacement of adsorbed atoms by a more noble metal, the thus-deposited metal film is limited to a single monolayer. Consequently forming a shell with layer thicknesses greater than one monolayer is not feasible. A recent approach which has been developed in attempting to overcome this limitation involves diffusion-controlled hydrogen reduction of noble metal ions onto a less noble core. This has been disclosed, for example, by Y. Wang, et al. in “Preparation Of Pd—Pt Bimetallic Colloids With Controllable Core/Shell Structures”, J. Phys. Chem. B, 101, p. 5301 (1997) which is incorporated by reference as if fully set forth in this specification. However, this method involves the use of organic solvents which severely inhibit the catalytic activity of the deposited material. Another issue is that residue from the solvent itself is difficult to remove after synthesis.